Device for determining the energy of charged particles

ABSTRACT

A device for determining the energies of charged particles, as Auger electrons, comprises at least one durved or domeshaped electrode biased to converge the paths of particles which are emitted by a particle source into a solid angle spanned by said electrode. The electrode is convex toward the particle source and cooperates with a similar, fairly closely spaced second electrode, and with a bias source connected to said electrodes to produce a particle accelerating field between them, to focus particles of a predetermined energy into a particle detecting device.

United States Patent 1 Staib [451 Sept. 18, 1973 DEVICE FOR DETERMININGTHE ENERGY 0F CHARGED PARTICLES [76] Inventor: Philippe G. Staib,Mannhardtstrasse 3, Munich, Germany [22] Filed: Jan. 24, 1972 [21] Appl.No.: 219,934

[30] Foreign Application Priority Data Jan. 25, l97l Germany P 2l Q3306.7

52 us. Cl. 250/305, 250/310 51 int. Cl. n01; 37/26 [58] Field of Search250/495 AE, 49.5 PE,

[56] References Cited OTHER PUBLICATIONS Photoelectron Spectroscopy ofthe Rare Gases, Samson, The Physical Review, Vol. 173, September 1968,pp. 80-85.

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. ChurchAttorney-Flynn & Frishauf [57 1 ABSTRACT A device for determining theenergies of charged particles, as Auger electrons, comprises at leastone durved or domeshaped electrode biased to converge the paths ofparticles which are emitted by a particle source into a solid anglespanned by said electrode. The electrode is convex toward the particlesource and cooperates with a similar, fairly closely spaced secondelectrode, and with a bias source connected to said electrodes toproduce a particle accelerating field between them, to focus particlesof a predetermined energy into a particle detecting device.

7 Claims, 6 Drawing Figures PATENTEDSEP 1 8l973 I SHEET 2 0F 5 Fig.4

PATENTED SEP] 8. I973 SHEET ,3 [1F 5 PATENTED SEP] 8 I975 SHLET N [If 5PATENTEB SEP] 8 I973 SHEET 5 BF 5 DEVICE FOR DETERMINING THE ENERGY OFCHARGED PARTICLES BACKGROUND OF THE INVENTION The present inventionrelates to a device for determining the energy of charged particles,especially electrons, emitted from a source of particles into a givensolid angle, with an arrangement of electrodes which includes at leastone electrode, spanning the solid angle, curved and perforated andconnected to one terminal of a bias source, and which generates anelectrical field that produces a convergent beam from the particlesdiverging from the source; with a particledetecting device, arranged onthe side of the electrodes away from the source of particles, at thepoint of smallest cross-section of the beam of particles; and with adiaphragm arranged between the source of particles and theparticle-detecting device, which prevents particles from the sourcereaching the particle-detecting device on a straight orbit.

An energy analyzer of this type is described in the journal AppliedPhysics Letters, Vol. 16, no.9, May 1 1970, pages 348 to 351. Itrepresents a further development of the so-called LEED retarding-fieldapparatus and, like it, has the advantage over deflecting-field systemenergy analyzers, as used in mass spectrometers, of substantially highertransmission.

The energy analyzer described in the above-quoted publication consistsof a high-resolution opposing-field section with twospherical-cap-shaped grids, concave in relation to the source ofparticles, having a separation of 6.25 cm and an average radius ofapproximately 40.

cm. This geometry comprehends a solid angle of 0.08

17 with a source area of 2 cm. Behind the opposingfield section apotential trough is produced which operates only on the low-energyelectrons (e 10 eV). By this means, noise is greatly reduced bycomparison with the LEED opposing-field apparatus, which represents ahigh-pass filter, because most of the faster electrons (E 10 eV) are notdetected. The depth of the potential trough should not be too great, inorder that not too many electrons with an energy of a few eV reach thedetecting system, but it should be deep enough for a satisfactorypercentage of low-energy electrons to be detected.

SUMMARY OF THE INVENTION An object of the present invention is toimprove the resolving power and the transmission of the abovementioned,known energy analyzer, especially for small particle-energies.

According to the invention, this and other objects are achieved in thatthe curved electrode is convex in relation to the source of particlesand is so biassed in relation to a second perforated electrode, whichlies between it and the particle-detecting device, that there prevails.between these electrodes an electrical field which accelerates theparticles. I

The second electrode should preferably be curved concentrically with thefirst electrode.

Anenergy analyzer of this kind can be arranged with advantage in theorbit of the rays behind a LEED retarding-fleld apparatus.

The device according to the invention is especially but not exclusivelysuitablefor determining the energy of Auger electrons and photoelectronsin the context of non-destructive chemical analysis by electronspectroscopy.

Other objects, features and advantages of the inven tion will beapparent from the following detailed description of preferredembodiments thereof, reference being made to the accompanying drawings,in which;

FIG. 1 is a diagrammatic representation of a method by which theinvention is to be performed which is especially suitable fordetermining the energy of lowenergy electrons;

FIG. 2 is a diagrammatic representation of a modification of the methodaccording to FIG. 1;

FIG. 3 is adiagrammatic representation of a further method by which theinvention is to be performed;

FIG. 4 is a diagrammatic representation of various cross-sections ofbeams of radiation in a plane P in FIG. 1;

FIG. 5 is an axial section which shows the relations in the area of thesmallest cross-section of the beam in the device according to FIG. 1 or3 and FIG. 6 is a diagrammatic perpsective view of a method by which theinvention is to be performed with cylindrical geometry.

FIG. 1 shows in diagram form an energy analyzer according to theinvention, which consists essentially of two concentric,sperical-cap-shaped wire-mesh grids 10,12, arranged in separation(proportion of separation to average radius preferably more than 0.01,e.g. 0.03 0.5; especially 0.05 to 0.1) between a source of particles,e.g. electrons, placed in FIG. 1 to the left of the grids and aparticledetecting device, not shown in FIG. 1, arranged to the right ofthe grids, preferably a secondary electron multiplier. If an energyspectrum is to be collected, the particle-detecting device can bemoveable along the axis 14 of the system so that its entry diaphragm canbe brought into various planes perpendicular to the axis, of which oneplane P is shown in the drawing. Another possibility for the collectionof an energy spectrum consists in altering the voltage U at the grid 12while keeping the particledetecting device fixed.

In what follows, it should be assumed for the sake of simplicity thatthe particles, whose energy is to be determined are electrons. Thepresent invention is of course also applicable to other chargedparticles, e.g. positive ions and in this case the signs simply have tobe reversed.

The grid 10 is in practice at the potential of the source of particles,while the grid 12, by a positive bias U is biassed in relation to thesaid grid 10 which is grounded. The bias U is large in comparison withthe voltage V corresponding to the energy E eU (e elementary charge) ofthe electrons and, in the analysis of low-energy electrons with enerigesof a few tenths of an eV, amounts to e.g. about 10 V.

Through the voltage U between the two grids l0, 12, the inner space ofthe spherical mesh is related as a sphere with the refraction index nQua/6U)" The relation between the angle of incidence is, and

the angle of reflection d, is determined by the optical refraction lawsin (b, n sin (b,

Where U U n alters very quickly depending on U and where U 0 itapproaches infinity, and this corresponds to a large dispersion ofenergy on the symmetrical axis 14. In FIG. 1a few refracted rays areshown for various values of U/U and for the inner and outer peripheralrays. The paraxial area is screened off by a circular disc 16. Thespherical aberration is small where values of U/U 0.01 and the beamsformed by monoenergetic electrons intersect the symmetrical axis 14within a small area. The combination of these two properties, namelyhigh dispersion of energy and sharp focus images, is highlyadvantageous.

The energy of the detected electrons depends on the position of theentry aperture of the radiation detecting device on the axis 14 and onthe applied voltage U This will be examined in more detail later.

The resolving power of the energy analyzer according to FIG. 1 isprincipally determined by the diameter of the source and the sphericalaberration. The spherical aberration can be excluded if the mesh gridsare given the form not of a spherical cap but of a cartesian oval. Theequation for an area of this kind reads in the system of coordinatesaccording to FIG. 2

When a grid of this form is used, the resolving power is then primarilydetermined by the diameter of the source and by coma error. Estimateshows that a resolving power of the order of 1 percent can be obtained.

In FIG. 2 the source of radiation is marked 18 and the entry diaphragmof the radiation-detecting device 20.

As shown in FIG. 3, the present energy analyzer can be combined with aLEED retarding-field apparatus. Between the source of electrons l8 andthe grids 10,12, represented in the form of a sperical cap, twoconcentric, spherical-cap-shaped grids 22, 24 are correspondinglyarranged, concave in relation to the source, between which aparticle-retarding field is generated. The space between grid 10 andgrid 24 is radially cut off by a truncated-cone-shaped (or cylindrical)grid 26. In FIG. 3, by way of example, the biasses for the variouselectrodes are given when the system is to be used for the analysis ofelectrons with energies of the order of approximately 0.] to 10 eV.

A mesh lens causes dispersion of the beam of particles. This dispersionis practically only observable at electrode 24 of the retarding-fieldsection formed by electrodes 22 and 24, because the energies of theparticles extend to zero here on account of the retarding effect, andthe angle of dispersion increases rapidly as the energy of the particlesdecreases. Owing to the high value of the refraction index of the meshlens, formed by the mesh electrodes 10,12, for low-energy electrons,these angles of dispersion are very sharply reduced after refraction.The cross-sections in the plane P (FIG. 1) for an arrangement of thekind shown in FIG. 3, taking the dispersion into account, are shown inFIG. 4 for the inner peripheral rays (U 100 V).

For small particle-energies (approx. 0.1 eV after retarding), thesecross-sections are circles, whose radii increase with the energy. Whenthe energy increases further, the cross-sections of the rays becomeringshaped, as shown by the cross-sections for 1.0 eV, 2.0 eV and 5.0 eVin FIG. 4. The detecting device has, correspondingly, an entry diaphragm20 (FIG. 5) with a circular aperture, the centre of which lies on thesymmetrical axis 14 of the rotation-symmetrical system. Where adiaphragm with an aperture radius r is used, all electrons up to anenergy of B are detected, as shown in the following equation:

E (r/R) eU In this equation, R represents the average radius of theelectrodes 10,12. With higher energies the crosssections increase andthe measured intensity decreases. Particles whose energy is so greatthat the inner peripheral ray determined by the disc 16 no longer passesthrough the diaphragm aperture, are not detected.

If spherical-cap-shaped mesh electrodes 10,12 are used, the resolvingpower can be improved by a disc 28, arranged in front of the entrydiaphragm 20 of the detecting aperture, the exact position of which canbe seen from FIG. 5.

The invention is not confined to rotationsymmetrical geometries. It isalso, for example, possible to work with a cylindrical symmetry, asshown in FIG. 6. InFIG. 6, the corresponding parts are designated withthe same reference numbers as in FIG. 2, with the addition of an accent.The source of radiation 18', the diaphragm 16 for screening off theparaxial rays, and the entry diaphragm 20 of the radiation-detectingdevice are elongated and strip-shaped, while the meshelectrodes 10', 12'are cylindrical in form. Depending on the type of modification wanted,the electrodes in a plane intersecting the axis of the cylinder atrightangles can have the form of a circle, an ellipse, a parabola or asection of the above-mentioned cartesian oval.

The second grid electrode 12 can also be'arranged in greater separationbehind the grid electrode 10 and in certain circumstances even bereduced to a relatively small, flat grid electrode or diaphragm directlyin front of the particle-detecting device (e.g. multiplier orcapture-apparatus).

Various modifications of the above described embodiments of theinvention will be apparent to those scilled in the art, and it is tounderstood that such modifications can be made without departing fromthe scope of the invention if they are within the spirit and tenor ofthe accompanying claims.

I claim:

1. A device for determining the energy distribution of chargedparticles, especially electrons, emitted from a source of particles in agiven solid angle, comprising:

an arrangement of electrodes which includes potentials least a firstperforated electrode (10,10)

spanning said solid angle, which electrode is convex towards said sourceof particles (18), and a second perforated electrode (12,12'), likewiseconvex towards said source, radially spaced from said first electrode onthe side of said first electrode farthest from said source;

electric potential bias means for applying otentials to said first andsecond electrodes such as to generate an electric field that causes saidparticles diverging.

from said source to converge after passing through said first and secondelectrodes;

a particle detecting device located on the median of the arrangement ofsaid electrodes and said source at a greater distance from said sourcethan said electrodes, said particle detecting device having an entrancediaphragm with an aperture of such restricted size and shape as toexclude particles converging on said median at more than a predeterminedsmall distance behind said diaphragm;

a baffle (16,28) centered on said median and located between said sourceof particles and said diaphragm, said baffle being of a size and shapeat least approximately including the direct projection from said sourceof said diaphragm aperture, so that particles with little or nodivergence from said median are blocked from said particle detectingdevice; and

scanning means for enabling said particle detecting device to scan overa range of particle emission energies of said source.

2. A device as defined in claim 1 in which the curvature of said firstand second electrodes is concentric,

3. A device as defined in claim 1 in which said scanning means includesmeans for moving said particle detecting device towards or away fromsaid source along said median.

4. A device as defined in claim 1 in which said median is an axispassing through said source and said first and second perforatedelectrodes are rotationsymmetrical with respect to said axis, and inwhich, further, said diaphragm aperture of said particle detectingdevice is circular and located so that said axis passes therethrough.

5. A device as defined in claim 4 in which said bafile is a disc (28)disposed between said second electrode and said diaphragm.

6. A device as defined in claim 1 in which the proportion of the spacingof said first and second perforated electrodes to the average radius ofcurvature of said electrodes is between 0.0] and 0.5.

7. A device as defined in claim I in which said scanning means includesa retarding-field analyzer disposed between said source of particles andsaid first perforated electrode, said retarding-field analyzercomprising at least a third and a fourth perforated electrodes (22,24)concave towards said source and connected to an electric bias source soas to produce a particleretarding electric field between said third andfourth electrodes, and in which further the space between saidretarding-field analyzer and said first perforated electrode (10,10) isbounded by a fifth electrode (26) of cylindrical or truncated-coneshape.

1. A device for determining the energy distribution of chargedparticles, especially electrons, emitted from a source of particles in agiven solid angle, comprising: an arrangement of electrodes whichincludes potentials least a first perforated electrode (10,10'')spanning said solid angle, which electrode is convex towards said sourceof particles (18), and a second perforated electrode (12,12''), likewiseconvex towards said source, radially spaced from said first electrode onthe side of said first electrode farthest from said source; electricpotential bias means for applying otentials to said first and secondelectrodes such as to generate an electric field that causes saidparticles diverging from said source to converge after passing throughsaid first and second electrodes; a particle detecting device located onthe median of the arrangement of said electrodes and said source at agreater distance from said source than said electrodes, said particledetecting device having an entrance diaphragm (20) with an aperture ofsuch restricted size and shape as to exclude particles converging onsaid median at more than a predetermined small distance behind saiddiaphragm; a baffle (16,28) centered on said median and located betweensaid source of particles and said diaphragm, said baffle being of a sizeand shape at least approximately including the direct projection fromsaid source of said diaphragm aperture, so that particles with little orno divergence from said median are blocked from said particle detectingdevice; and scanning means for enabling said particle detecting deviceto scan over a range of particle emission energies of said source.
 2. Adevice as defined in claim 1 in which the curvature of said first andsecond electrodes is concentric.
 3. A device as defined in claim 1 inwhich said scanning means includes means for moving said particledetecting device towards or away from said source along said median. 4.A device as defined in claim 1 in which said median is an axis passingthrough said source and said first and second perforated electrodes arerotation-symmetrical with respect to said axis, and in which, further,said diaphragm aperture of said particle detecting device is circularand located so that said axis passes therethrough.
 5. A device asdefined in claim 4 in which sAid baffle is a disc (28) disposed betweensaid second electrode and said diaphragm.
 6. A device as defined inclaim 1 in which the proportion of the spacing of said first and secondperforated electrodes to the average radius of curvature of saidelectrodes is between 0.01 and 0.5.
 7. A device as defined in claim 1 inwhich said scanning means includes a retarding-field analyzer disposedbetween said source of particles and said first perforated electrode,said retarding-field analyzer comprising at least a third and a fourthperforated electrodes (22,24) concave towards said source and connectedto an electric bias source so as to produce a particle-retardingelectric field between said third and fourth electrodes, and in whichfurther the space between said retarding-field analyzer and said firstperforated electrode (10, 10'') is bounded by a fifth electrode (26) ofcylindrical or truncated-cone shape.